This sounds like /var/log may be filling.
You might check jounralctl or clean out logs with
sudo journalctl --vacuum-time=1s
and see if that space used is reduced.
I pretty much always think a fresh install is best.
I, too, suffer from wanting to avoid it. But the reality is, it is never as bad as I fear and it reduces clutter, clears out junk I have forgotten about and gives a nice fresh clean start. This reduces conflicts that build up, as well.
I do back up personal files, docs, music, etc on an external drive.
For reinstalling software, I use this method:
Sorry for the delays and short messages: It is hectic here...